Reverts asset removal from commit 292378735251afe221f77c097095c859dc49caf5.
File icons have been updated for consistency and a StripThumbnail method added to File for icon display in gridfield instances (such as file selection interface)
git-svn-id: svn://svn.silverstripe.com/silverstripe/open/modules/sapphire/trunk@39001 467b73ca-7a2a-4603-9d3b-597d59a354a9